Feat: Download a specific version of schemas by maven plugin

[danielpetisme]

This PR aims at providing a way to download a specific version of a subject rather than the last one.

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[danielpetisme]

Feel free to correct me, but from my understanding the current DownloadSchemaRegistryMojoTest does not test anything...
There is no mention of .execute to trigger the mojo behavior.
So the very first step was to baseline the current expected behavior with a running test.

[danielpetisme]

I proposed a first very pragmatic approach.
As fore now, the subjectPatterns argument is mandatory for the MOJO config
https://github.com/confluentinc/schema-registry/blob/master/maven-plugin/src/main/java/io/confluent/kafka/schemaregistry/maven/DownloadSchemaRegistryMojo.java#L52

To prevent breaking changes I propose to define a subejct's version with a :<int> suffix on the pattern (it's an option I'm familiar with Gradle.
The following configurations are valid

<subjectPatterns>
  <param>^TestSubject000-(key|value)$</param>
  <param>^TestSubject001-(key|value)$:42</param>
</subjectPatterns>

if the separator : is not present, the MOJO will consider the user is expecting the latest version of the subject (just as it work right now). If : is present then the second part of the string will be considered as the a version (casted to integer).
When a version is defined, it will be associated to all the subjects matching the pattern (in the above example, Maven will download the version 42 for both TestSubject001-key and TestSubject001-value subjects.

The code can be improved but I really wanted to prototype a behavior to open the discussion.

[danielpetisme]

As explained the main ambition of this prototype was to not introduce a breaking change. However I must admit the Developer experience is not good with this solution.
The best approach would be to proposes new properties to let the user be more explicit, like

<subjects>
  <subject>
   <!--- full explicit -->
    <name>TestSubject000-key</name>
    <version>42</version>
    <format>avro</format>
  </subject>
  <subject>
   <!--- version: latest, format: default/avro -->
    <name>TestSubject000-key</name>
  </subject>
  <subject>
    <!--- name and pattern would be mutually exclusive, format: default/avro -->
    <pattern>TestSubject100-(key|value)</pattern>
    <version>42</version>
  </subject>
  <subject>
    <!--- version: latest, format: default/avro -->
    <pattern>TestSubject100-(key|value)</pattern>
  </subject>
<subjects>

WDYT?

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