feat: add DEFAULT clause

[Elia-Renzoni]

@ngjaying
This PR introduces support for the DEFAULT clause. At the moment, the implementation covers only the following data types:

• STRING
• FLOAT
• BOOLEAN
• BIGINT

If this approach looks good to you, I can extend the support to all the other available data types.

From a design perspective, and to avoid large refactors, the DEFAULT clause is currently enabled only when STRICT_VALIDATION is set to true. This means the feature relies on the preprocessor to work properly.

Let me know your thoughts or if you'd like me to adjust anything.

Related Issue: #2293

[Elia-Renzoni]

There is a minor issue in the CI pipeline related to a unit test. However, it does not appear to be caused by the changes in this PR. I ran the same codebase without my modifications and the error is still reproducible, so we can conclude that it is unrelated to this change?

Let me know if you already have context on it.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.66%. Comparing base (1c2f927) to head (d655909).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/ast/sourceStmt.go	48.24%	44 Missing ⚠️
internal/xsql/parser.go	70.83%	6 Missing and 1 partial ⚠️
internal/processor/stream.go	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4025      +/-   ##
==========================================
+ Coverage   70.63%   70.66%   +0.02%     
==========================================
  Files         459      459              
  Lines       53324    53449     +125     
==========================================
+ Hits        37665    37765     +100     
- Misses      12719    12749      +30     
+ Partials     2940     2935       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.