ENH: add pluggable e-log integration and /v1/elog routes

[YektaY]

Description

• Introduces an ElogAdapter plugin contract so other labs/groups within slac can swap in their own
  e-log; ships an ElogPlusAdapter targeting elog-plus.
• Adds /v1/elog/config, /v1/elog/logbooks, /v1/elog/logbooks/{id}/tags,
  and POST /v1/elog/entries. The author on each entry is stamped from the
  API key's appName — never trusted from the client.
• Env-configurable outbound proxy (SQUIRREL_ELOG_PROXY_URL) for control
  rooms that must egress through a gateway.
• Feature is off by default; GET /config returns enabled=false when
  SQUIRREL_ELOG_PROVIDER is unset, and the frontend hides the button.

Motivation

Adds in ability to post to the e-log, which was a request from one of our test facilities.

Where Has This Been Documented?

Documentation on how to add a new e-log plugin was added.

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions