[Feat]: make dependency starlette optional

[sll521]

Is your feature request related to a problem? Please describe.

we have a project used a lower version starlette>=0.37.2,<0.38.0, but the a2a-sdk use starlette>=0.46.2, confict! My current solution is to directly modify the dependent version using the source code, but upgrading later will be troublesom. If there is a better way, please advise.

Describe the solution you'd like

I hope the SDK can provide a way to solve my problem, below are my some idea:
way 1. use a lower verson of starlette,may cause incompatibility issue
way 2. make starlette optional, then offer an abstract application do not dependent specific web framework，let user extend

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[klauswong]

Facing the same issue where the dependencies conflict with Chainlit requirements
As a tmp solution, we forked the a2a-python and lowered the version requirement
klauswong@f5f7263

[darkhaniop]

@holtskinner, do you mind if I try implementing the second solution instead (or after #193 if it gets merged in the meantime)?

way 2. make starlette optional, then offer an abstract application do not dependent specific web framework，let user extend

If we opt for "way 2," we can make both starlette and the newly added FastAPI frameworks optional, so that downstream projects only add dependencies they actually use, instead of carrying all possible frameworks.

This could be a better approach. What do you think?

[holtskinner]

@darkhaniop Go ahead and make a PR for it