[Feature Support]: Update CLI to enable hash-bin delegations under custom delegations

[srinjoydutta03]

Description

Introduced support for nested hash-bin delegations under custom delegations in the RSTUF CLI. The changes add interactive prompts to configure nested bins, update the delegation metadata to store bin information, and clarify the signing method restrictions for nested bins. Additionally, the delegation metadata display now includes the number of nested bins for each delegation.

Nested hash-bin delegation support

• Added _prompt_nested_bins function to prompt users for the creation and number of nested hash-bin delegations under a custom delegation. The number of bins is restricted to powers of two between 2 and 4096.
• Updated _configure_delegations to use _prompt_nested_bins, pass the bin count to _expiry_prompt, and store the bin count in the delegation's metadata (x-rstuf-num-bins).

Signing method restrictions

• Updated the signing method selection and user-facing documentation to clarify that only global online key (existing) is supported for both custom roles and their nested bins when hash-bin delegations are used. Offline keys are not supported for nested bins as of now.

Delegation metadata and display

• Modified the delegation metadata table to include a "Nested Bins" column, showing the number of bins for each delegation if present.

Expiry prompt improvements

• Enhanced _expiry_prompt to accept an optional num_bins parameter, updating the prompt messaging when nested bins are being configured.

Related Issue: repository-service-tuf/repository-service-tuf#876

@kairoaraujo

[MVrachev]

Nice work, but keep in mind this feature most likely will require multiple reviews, at least from me.

I noticed that during the bootstrap process the flow is the following:

1. hash bin or custom delegation -> choose custom
2. name of custom delegation
3. Do you want hash‑bin delegations under this 'project-a' delegation? -> yes
4. Number of bins
5. Please enter days until expiry for 'project-a' role and its nested roles
6. Please enter path

I think it's better if the user fully configures the necessery fields for the custom delegated role and in the end we mention something to the user like:

Delegated target role <NAME_OF_ROLE> configured
Do you want to add a hash bin delegations for that role?

and then after the configurmation continue.

@kairoaraujo do you agree here?

Also, add a new line between the Info here and the choice for online key:

[MVrachev]

Suggested change

	" role but they are always signed with the global online key"
	" role, but they are always signed with the global online key"

[srinjoydutta03]

Noted, I'll fix this!

[srinjoydutta03]

Nice work, but keep in mind this feature most likely will require multiple reviews, at least from me.

I noticed that during the bootstrap process the flow is the following:

1. hash bin or custom delegation -> choose custom
2. name of custom delegation
3. Do you want hash‑bin delegations under this 'project-a' delegation? -> yes
4. Number of bins
5. Please enter days until expiry for 'project-a' role and its nested roles
6. Please enter path

I think it's better if the user fully configures the necessery fields for the custom delegated role and in the end we mention something to the user like:

Delegated target role <NAME_OF_ROLE> configured
Do you want to add a hash bin delegations for that role?

and then after the configurmation continue.

@kairoaraujo do you agree here?

Also, add a new line between the Info here and the choice for online key:

Yes, this simplifies things more. We can do this.