feat(js/core): Add dynamic-action-provider support

[ifielker]

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[ssbushi]

Not a strong opinion but looks like the delta between the old and new APIs is only these 4 lines.

Do we need to deprecate the old API instead of providing dynamic actions as an add on?

createMcpHost({...}).withDynamicActions({ai})

This is not consistent with our defineBlah({ai, ...}) APIs but it will avoid deprecating the old one?

[ifielker]

It requires the ai parameter and it also makes the name field of the options a required field. It's a breaking change to createMcpHost.