feat: Add customizable instance settings

[kostyaplis]

Description

Inspired by #119. Added with readme, example and requested changes.

Allow setting individual parameters for cluster instances

Motivation and Context

We needed to add additional read only replica, for reporting needs. Some small instance type was enough for this task, however module allows only to specify number of nodes which share same parameters. We also needed to customize instance name and failover priority - promotion_tier (we do not want to failover to small instance).

Breaking Changes

Requires TF >= v0.12
It is 100% compatible with previous versions.
If you specify variable in a module call:

replica_count = 3

instances_parameters = [
{
  instance_name = "db-a"
  instance_type = "db.r5.large"
  instance_promotion_tier = 1
},
{
  instance_name = "db-b"
  instance_type = "db.t3.medium"
  instance_promotion_tier = 10
},
{
  instance_promotion_tier = 15
}
]

Customised setting will be used if they are set. If settings are missing module will generate like in previous version name, promotion_tier and will use var.instance_type for all nodes.

How Has This Been Tested?

Tested on our AWS Dev account.
Tested cases:

• create brand new cluster without instances_parameters set
• create brand new cluster with instances_parameters set
• adding instances_parameters to an existing cluster
• removing instances_parameters from an existing cluster

[antonbabenko]

Resolved in #146

v2.26.0 has been just released.

[github-actions]

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