Add conn_attrs flag to admin db commands

[Shaddoll]

What changed?

• Introduce StringMap type to parse key=val CLI input into string map
• Add conn_attrs flag to admin db commands and use it for SQL database connection

Why?
To address #2777. Docstore will use conn_attrs for database connection.

How did you test it?
local test, new unit test

zijian@zijian-C02XW43AJGH6 cli % ./cli adm shard d --conn_attrs "a=b" --shard_id 0 --db_port 9042 --conn_attrs cd --db_type mysql 
Incorrect Usage: invalid value "cd" for flag -conn_attrs: should be in 'key=value' format

NAME:
   cadence admin shard describe - Describe shard by Id

USAGE:
   cadence admin shard describe [command options] [arguments...]

OPTIONS:
   --db_type value                 persistence type. Current supported options are [mysql postgres cassandra] (default: "cassandra")
   --db_address value              persistence address (right now only cassandra is fully supported) (default: "127.0.0.1")
   --db_port value                 persistence port (default: 9042)
   --db_region value               persistence region
   --username value                persistence username
   --password value                persistence password
   --keyspace value                cassandra keyspace (default: "cadence")
   --db_name value                 sql database name (default: "cadence")
   --encoding_type value           sql database encoding type (default: "thriftrw")
   --decoding_types value          sql database decoding types (default: "thriftrw")
   --tls                           enable TLS over cassandra connection
   --tls_cert_path value           cassandra tls client cert path (tls must be enabled)
   --tls_key_path value            cassandra tls client key path (tls must be enabled)
   --tls_ca_path value             cassandra tls client ca path (tls must be enabled)
   --tls_enable_host_verification  cassandra tls verify hostname and server cert (tls must be enabled)
   --conn_attrs value              a key-value set of sql database connection attributes (must be in key=value format) (default: map[a:b])
   --shard_id value                The Id of the shard to describe (default: 0)
   
invalid value "cd" for flag -conn_attrs: should be in 'key=value' format

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build ad5ffe24-baec-4ba6-a5dd-dd215dc17b41

• 18 of 37 (48.65%) changed or added relevant lines in 3 files are covered.
• 47 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.02%) to 60.288%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tools/cli/adminCommands.go	0	8	0.0%
tools/common/flag/flag.go	13	24	54.17%

Files with Coverage Reduction	New Missed Lines	%
client/history/client.go	2	44.78%
client/history/metricClient.go	2	49.43%
common/task/weightedRoundRobinTaskScheduler.go	2	89.12%
service/history/handler.go	2	46.67%
service/history/queue/timer_queue_processor.go	2	58.77%
common/cache/lru.go	3	90.73%
service/frontend/workflowHandler.go	4	58.35%
service/history/historyEngine.go	9	71.18%
common/persistence/cassandra/cassandraPersistence.go	21	55.47%

Totals
Change from base Build 8877bb7e-1e22-4212-9252-ad05b7afefd5:	0.02%
Covered Lines:	88600
Relevant Lines:	146962

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💛 - Coveralls

[longquanzheng]

Just minor comments

[longquanzheng]

LGTM