Choose a canonical names for services

[omus]

Right now @service supports allowing the user to define the casing used for the module name:

AWS.jl/src/AWS.jl

Lines 113 to 116 in ca6b550

	# All of the examples below are valid!
	@service Secrets_Manager
	@service SECRETS_MANAGER
	@service sECRETS_MANAGER

If we implement #435 we should probably choose a single canonical spelling for each service instead of having the user be able to specify their own.

I think going with the standard Julia module naming convention is the correct approach which would result in:

ServiceId	Proposed Module Name	Current Module Name
S3	S3	s3
Secrets Manager	SecretsManager	secrets_manager
ACM PCA	ACMPCA	acm_pca

[omus]

Proposal for an backwards compatible implementation:

1. When generating the high-level API file names keep the casing as defined by the ServiceId and keep the whitespace.
2. Generate a mapping of lowercase service names to the high-level API file name that have entries using underscore or no space separator (e.g. "secretsmanager" => "Secrets Manager.jl", "secrets_manager" => "Secrets Manager.jl")
3. When using @service lower case the specified module name and use the mapping to determine the service file name.
4. If the module name passed into @service (prior to lower casing) does not match the service name (the service file name with spaces and the extension removed) then emit a deprecation warning

This should keep the current behaviour and also allow the use of the new proposed module names