Added `w.clusters.commands(cluster_id).run('return [1,2,3]')` utility

[nfx]

Changes

This PR refactors the wrapper around Command Execution API, which was used by dbutils.fs.mounts() and other cluster-proxied dbutils into a dedicated mixin.

Usage:

cluster_id = env_or_skip("TEST_DEFAULT_CLUSTER_ID")
ce = w.clusters.commands(cluster_id=cluster_id)
rows = ce.run('return spark.sql("SHOW DATABASES").collect()')
assert len(rows) > 0

and results:

Tests

• [x] make test run locally
• [x] make fmt applied
• [x] relevant integration tests applied

[codecov-commenter]

Codecov Report

Patch coverage is 75.41% of modified lines.

Files Changed	Coverage
databricks/sdk/mixins/compute.py	72.50%
databricks/sdk/dbutils.py	100.00%

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