Operator extra links per attempt (draft B): the link opts in

[1fanwang]

TL;DR — operator extra links don't work per attempt (#71471). After a retry the button
under attempt 1 opens attempt 2's logs. It hits every bundled provider linking to per-attempt
logs: EMR, Glue, Databricks, Dataproc, Livy.

Draft B of three for the same bug — pick a shape and I'll finish it and close the others.
Recommended: C (#71522). See the issue for why; A is the fallback if this needs to ship on the 3.2 line.

	Approach	Trade
A	#71518 — a row per attempt in XCom	smallest; core owns a key convention for every link
B	this PR — the link opts in	no extra rows when unneeded; changes the serialized DAG format
C	#71522 — the task state store	nothing about the clear moves; 3.3+ only

How

BaseOperatorLink gains keeps_a_link_per_attempt and xcom_key_for_try. A link that sets
the flag gets a row per attempt, exempt from the retry clear; everything else behaves as it
does today.

The flag has to survive DAG serialization, and that is the cost. The api-server never sees a
provider's link class — it rebuilds every link as XComOperatorLink from a {name: xcom_key}
mapping carrying a string and nothing else. That mapping now carries
{"key": ..., "per_attempt": true} for opted-in links and stays a bare string otherwise, so
existing serialized DAGs read back unchanged.

Testing

Read path against a real metastore on 3.2.2, rows present for both attempts, asking an
opted-in link and a plain one.

Raw output

$ AIRFLOW__DATABASE__SQL_ALCHEMY_CONN=sqlite:///af.db python b_test.py
opted-in  try 1 -> https://logs/attempt-1
opted-in  try 2 -> https://logs/attempt-2
not opted try 1 -> https://logs/attempt-2

Line 3 is the before state on the same path: without the flag, attempt 1 still returns the
latest attempt's URL. Lines 1 and 2 are the same read with it set.

Key derivation:

$ python -c "from airflow.serialization.definitions.operatorlink import XComOperatorLink; ..."
default  try1 -> _link_X                 | try2 -> _link_X
opted-in try1 -> _link_attempt_1__link_X | try2 -> _link_attempt_2__link_X

Serialization round-trip, the risky part:

encoded: {'plain': '_link_P', 'opted': {'key': '_link_O', 'per_attempt': True}}
  plain: key=_link_P per_attempt=False  (legacy string, unchanged)
  opted: key=_link_O per_attempt=True

[1fanwang]

Going with #71522 instead: the task state store is where cross-retry state already lives, so the XCom clear stays absolute rather than gaining an exemption. Closing this one.