feat(core): add bureau shutdown event support

[Archento]

Proposed Changes

Add support for shutdown tasks within the agent Bureau

Linked Issues

Closes #90

Types of changes

• Bug fix (non-breaking change that fixes an issue).
• New feature added (non-breaking change that adds functionality).
• Breaking change (fix or feature that would cause existing functionality to stop working as expected).
• Documentation update.
• Something else (e.g., tests, scripts, example, deployment, infrastructure).

Checklist

• I have read the CONTRIBUTING guide
• Checks and tests pass locally

If applicable

• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease
• I have added/updated the documentation

Further comments

To verify please execute the following snippet:

from uagents import Agent, Bureau, Context

alice = Agent(name="Alice", seed="<seed>")
bob = Agent(name="Bob", seed="<seed>")

bureau = Bureau(
    agents=[alice, bob],
    port=8080,
    endpoint="http://localhost:8080/submit",
)

@alice.on_event("startup")
async def startup1(ctx: Context):
    ctx.logger.info(">>> Alice is starting up.")

@bob.on_event("startup")
async def startup2(ctx: Context):
    ctx.logger.info(">>> Bob is starting up.")

@alice.on_event("shutdown")
async def shutdown1(ctx: Context):
    ctx.logger.info(">>> Alice is shutting down.")

@bob.on_event("shutdown")
async def shutdown2(ctx: Context):
    ctx.logger.info(">>> Bob is shutting down.")

bureau.run()