Add parameter types for record accessors and place holders

[tagomoris]

Is your feature request related to a problem? Please describe.

Plugin parameters define string parameters that are used to:

• specify a key of records (to be turned into RecordAccessor)
• specify a dynamic parameter value (to be computed by extract_placeholders)

Those types of parameters are currently described as a string, and plugin authors have to explain what those are in the description. It's a hard thing to both plugin authors and users. If we can explain it as parameter types, the situation will be much clearer than now.

Describe the solution you'd like

Add 2 parameter types. Parameters of those types will be described clearly differently from :string in plugin documents.

record_accessor

Parameter type record_accessor is to instantiate a RecordAccessor object from the parameter value.

# definition
config_param :key, :record_accessor

# configuration
key $.author.name

 # usage
@key #=> RecordAccessor object
@key.call(record)

dynamic_string

Parameter type dynamic_string is to instantiate a new type object to call extract_placeholders with an argument chunk.

# definition
config_param :path, :dynamic_string

# configuration
path /path/to/stored/file.${tag}.%Y_%m_%d_%H.log

# usage
@path #=> DynamicString object
@path.call(chunk) #=> /path/to/stored/file.tag.2022_09_04.log

Describe alternatives you've considered

As far as I consider, there are no alternatives to solve this (documentation) problem.

Additional context

Documentation toolchains (like fluent-plugin-config-format) need to be updated when this idea is taken.

[callmetushar123]

Hey I would really love to solve this issue please assign it to me