feat: support explain syntax

[francis-du]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Support explain syntax like this:

explain select * from foo

Checklist

• Add explain to sql parser
• Add explain test to sql parser
• Handle explain in DataNode
• Handle explain in Frontend

Refer to a related PR or issue link (optional)

Closes #531

[francis-du]

@v0y4g3r Hi Lei, Could you review this PR, If you have time.

[v0y4g3r]

@v0y4g3r Hi Lei, Could you review this PR, If you have time.

Glad to help. We will finish the review asap.

[codecov]

Codecov Report

Merging #546 (af5dd98) into develop (0791c65) will decrease coverage by 0.10%.
The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           develop     #546      +/-   ##
===========================================
- Coverage    86.41%   86.30%   -0.11%     
===========================================
  Files          400      402       +2     
  Lines        50650    50886     +236     
===========================================
+ Hits         43770    43918     +148     
- Misses        6880     6968      +88     

Flag	Coverage Δ
rust	86.30% <60.00%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/datanode/src/instance.rs	34.05% <0.00%> (-1.02%)	⬇️
src/datanode/src/instance/sql.rs	81.39% <0.00%> (-3.98%)	⬇️
src/datanode/src/sql/create.rs	89.52% <0.00%> (ø)
src/frontend/src/instance.rs	71.89% <0.00%> (-1.42%)	⬇️
src/frontend/src/instance/distributed.rs	77.50% <0.00%> (-6.80%)	⬇️
src/query/src/datafusion/planner.rs	67.27% <0.00%> (-14.95%)	⬇️
src/query/src/sql.rs	95.31% <0.00%> (-1.21%)	⬇️
src/sql/src/statements.rs	80.62% <ø> (ø)
src/sql/src/statements/statement.rs	50.00% <0.00%> (+1.72%)	⬆️
src/sql/src/statements/explain.rs	57.14% <57.14%> (ø)
... and 26 more

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[v0y4g3r]

@francis-du Could you please fix the failing clippy check? You can test before push by running following command in your own machine:

cargo clippy --workspace --all-targets -- -D warnings -D clippy::print_stdout -D clippy::print_stderr

[francis-du]

@francis-du Could you please fix the failing clippy check? You can test before push by running following command in your own machine:

cargo clippy --workspace --all-targets -- -D warnings -D clippy::print_stdout -D clippy::print_stderr

cargo clippy validation passed locally

[francis-du]

I think the Clippy error comes from GitHub Action

[v0y4g3r]

LGTM