[#6578] docs(catalog): Add Fluss catalog support design

[xxubai]

What changes were proposed in this pull request?

This PR adds a design document for Apache Fluss catalog support in Gravitino.

The document covers:

• The proposed catalog-fluss module and catalog properties such as bootstrap.servers.
• Mapping Gravitino catalog, schema, table, and partition operations to Fluss metadata capabilities.
• Flink and Spark connector integration paths using native Fluss engine connectors for the data plane.
• Error handling, testing strategy, authentication, and authorization options.
• A recommended hybrid dual-layer authorization model using Gravitino built-in authorization and Fluss native ACLs.

Why are the changes needed?

Issue #6578 proposes supporting Apache Fluss catalog integration in Gravitino. Fluss is a streaming storage system for real-time analytics, and native catalog support would allow Fluss metadata to be managed through Gravitino's unified catalog, schema, table, partition, and governance APIs.

Fix: #6578

Does this PR introduce any user-facing change?

No. This PR only adds a design document.

The document proposes future user-facing concepts such as provider=fluss, bootstrap.servers, and Flink/Spark bypass property conventions, but does not implement them in this patch.

How was this patch tested?

Not run. This is a documentation-only design proposal.

[github-actions]

Code Coverage Report

Overall Project	65.58%	🟢
Files changed	No Java source files changed	-

Module	Coverage
aliyun	1.73%	🔴
api	47.13%	🟢
authorization-common	85.96%	🟢
aws	1.1%	🔴
azure	2.6%	🔴
catalog-common	10.2%	🔴
catalog-fileset	80.02%	🟢
catalog-glue	82.47%	🟢
catalog-hive	81.83%	🟢
catalog-jdbc-clickhouse	79.06%	🟢
catalog-jdbc-common	43.93%	🟢
catalog-jdbc-doris	80.28%	🟢
catalog-jdbc-hologres	54.03%	🟢
catalog-jdbc-mysql	79.23%	🟢
catalog-jdbc-oceanbase	78.38%	🟢
catalog-jdbc-postgresql	82.05%	🟢
catalog-jdbc-starrocks	78.27%	🟢
catalog-kafka	77.01%	🟢
catalog-lakehouse-generic	45.07%	🟢
catalog-lakehouse-hudi	79.1%	🟢
catalog-lakehouse-iceberg	86.98%	🟢
catalog-lakehouse-paimon	77.71%	🟢
catalog-model	77.72%	🟢
cli	44.51%	🟢
client-java	77.63%	🟢
common	48.75%	🟢
core	81.6%	🟢
filesystem-hadoop3	76.97%	🟢
flink	40.55%	🟢
flink-runtime	0.0%	🔴
gcp	14.2%	🔴
hadoop-common	10.39%	🔴
hive-metastore-common	46.83%	🟢
iceberg-common	55.24%	🟢
iceberg-rest-server	68.05%	🟢
integration-test-common	0.0%	🔴
jobs	66.17%	🟢
lance-common	23.88%	🔴
lance-rest-server	57.84%	🟢
lineage	53.02%	🟢
optimizer	82.87%	🟢
optimizer-api	21.95%	🔴
server	85.6%	🟢
server-common	70.01%	🟢
spark	32.79%	🔴
spark-common	39.09%	🔴
trino-connector	35.14%	🔴

[xxubai]

Can you take a look for this design? @roryqi @jerryshao

[jerryshao]

Sure, I will. Sorry for late response.