Add MongoDB driver handshake metadata

[alexbevi]

Summary

This PR identifies Burr in the MongoDB driver handshake, enabling server-side telemetry to attribute connections back to the library. MongoDB records this metadata at connection time, making it visible incurrentOp, Atlas performance advisor, and mongod logs.

What changes

A module-level constant is defined once in b_pymongo.py:

_DRIVER_INFO = DriverInfo(name="Burr", version=_VERSION)

This is injected at every MongoClient construction site and, for the case where a caller passes in an existing client, via append_metadata.

How it appears in the logs

On a MongoDB 4.4+ server, the isMaster/hello handshake carries the
following client metadata document (truncated to key fields):

{
  "driver": {
    "name": "PyMongo|Burr",
    "version": "4.9.2|0.42.0"
  },
  "platform": "CPython 3.10.3.final.0"
}

In mongod diagnostic logs this appears as:

{"t":{"$date":"..."},"s":"I","c":"NETWORK","id":51800,"ctx":"conn1","msg":"client metadata","attr":{"remote":"127.0.0.1:54321","client":"conn1","doc":{"driver":{"name":"PyMongo|Burr","version":"4.9.2|0.42.0"},...}}}

The PyMongo|Burr driver name lets operators distinguish Burr connections from bare PyMongo connections in server logs, Atlas metrics, and db.currentOp().

Testing

Six unit tests added in tests/integrations/persisters/test_b_pymongo_driver_info.py
(no live database required):

• _DRIVER_INFO has name="Burr" and version matching the installed package
• from_values() passes driver=_DRIVER_INFO to MongoClient
• from_values() does not override a caller-supplied driver
• __init__() calls append_metadata(_DRIVER_INFO) when the client supports it
• __init__() does not raise when the client lacks append_metadata (older pymongo)

No version constraint change is needed — DriverInfo is available since
pymongo 4.0, and append_metadata since 4.14; the existing unconstrained
"pymongo" dependency satisfies both.

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.