Deprecate the "Master" nomenclature

[tlfeng]

Is your feature request related to a problem? Please describe.
OpenSearch repository is going to replace the terminology "master"with "cluster manager".
issue: opensearch-project/OpenSearch#472, with the plan for its terminology replacement.

Although the existing usages with "master" will be supported in OpenSearch version 2.x to keep the backwards compatibility, please prepare for the nomenclature change in advance, and replace all the usages with "master" terminology in the code base.
All the OpenSearch REST APIs and settings that contain "master" terminology will be deprecated in 2.0, and alternative usages will be added.

Describe the solution you'd like
Replace the terminology "master" with "cluster manager".

When being compatible with OpenSearch 2.0:

• Replace "master" in code comment and internal variable / method / class name.
• Replace "master" in REST API request parameter and response
• Replace OpenSearch REST API that contains "master" with the provided inclusive REST API
  such as replacing GET _cat/master with "GET _cat/cluster_manager"

Specifically,
The "master" terminology in the REST API will be exposed to the user, so should be prioritized.
Impacted REST API: GET <endpoint>:9600/_plugins/_performanceanalyzer/metrics
Document: https://opensearch.org/docs/1.3/monitoring-plugins/pa/api/
Detail: Names of 3 Performance Analyzer metrics can be contained in an optional parameter of the API request, and a field of the API response: Master_PendingQueueSize , Master_Task_Queue_Time , Master_Task_Run_Time .

Describe alternatives you've considered
None.

Additional context

[dblock]

For 2.0, at a minimum please ensure that the plugin is not calling any deprecated APIs in core or another plugin, and confirm below, then remove the 2.0.0 label. If you have time, do the complete deprecation as described in this issue.

[sruti1312]

PA does not call any deprecated API's in core or another plugin.

[kkhatua]

To be picked by @@kaushalmahi12