[code-reflection] RFR: Support storing the code that builds the code model

[Mourad Abbay]

In this PR we allow the user to decide how to store the code model. The first option is `TEXT`, if this is selected, we store the textual representation of the code model. The second option is `CODE_BUILDR`, if this is selected, we store the code that builds the code model. All work done here is around the second option, because the first option is already supported.

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Commit messages:
 - Add option to indicate how to store code model

Changes: https://git.openjdk.org/babylon/pull/305/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=305&range=00
  Stats: 27 lines in 1 file changed: 13 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/babylon/pull/305.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/305/head:pull/305

PR: https://git.openjdk.org/babylon/pull/305