pandaproxy/sr: protobuf rendering: refactor string rendering

[IoannisRP]

Implements: CORE-8906

Refactored string rendering to use absl::CEscape instead of fmt debug format.

It is done for 2 reasons:

1. Debug format is not supported in earlier versions of fmt that we still use.
2. Make the intent more explicit.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.3.x
• v24.2.x
• v24.1.x

Release Notes

• none

[vbotbuildovich]

CI test results

test results on build#61451

test_id	test_kind	job_url	test_status	passed
gtest_raft_rpunit.gtest_raft_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/61451#0194bdb7-145c-4cf9-bbf3-947af84289a1	FLAKY	1/2
gtest_raft_rpunit.gtest_raft_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/61451#0194bdb7-145d-44a0-8364-de6a2945b3eb	FLAKY	1/2
rptest.tests.compaction_recovery_test.CompactionRecoveryTest.test_index_recovery	ducktape	https://buildkite.com/redpanda/redpanda/builds/61451#0194bdfb-83b1-46db-a935-d438943c01fe	FLAKY	1/2
rptest.tests.compaction_recovery_test.CompactionRecoveryTest.test_index_recovery	ducktape	https://buildkite.com/redpanda/redpanda/builds/61451#0194be00-6ef6-4d29-addd-c492087cbeb5	FLAKY	1/2
rptest.tests.controller_log_limiting_test.ControllerLogLimitMirrorMakerTests.test_mirror_maker_with_limits	ducktape	https://buildkite.com/redpanda/redpanda/builds/61451#0194be00-6ef4-415d-8c83-570c90f2f23b	FLAKY	1/2
rptest.tests.partition_movement_test.SIPartitionMovementTest.test_shadow_indexing.num_to_upgrade=0.cloud_storage_type=CloudStorageType.ABS	ducktape	https://buildkite.com/redpanda/redpanda/builds/61451#0194be00-6ef4-415d-8c83-570c90f2f23b	FLAKY	1/2
rptest.tests.scaling_up_test.ScalingUpTest.test_scaling_up_with_recovered_topic	ducktape	https://buildkite.com/redpanda/redpanda/builds/61451#0194be00-6ef4-415d-8c83-570c90f2f23b	FLAKY	1/2

[vbotbuildovich]

/backport v24.3.x

[vbotbuildovich]

/backport v24.2.x

[vbotbuildovich]

Failed to create a backport PR to v24.3.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-24992-v24.3.x-25 remotes/upstream/v24.3.x
git cherry-pick -x efdbf2351a

Workflow run logs.

[vbotbuildovich]

Failed to create a backport PR to v24.2.x branch. I tried:

git remote add upstream https://github.com/redpanda-data/redpanda.git
git fetch --all
git checkout -b backport-pr-24992-v24.2.x-181 remotes/upstream/v24.2.x
git cherry-pick -x efdbf2351a

Workflow run logs.