support table alias in join clause

[houqp]

Which issue does this PR close?

Read table alias from join clause.

Refactored out from #55.

Rationale for this change

Make #55 a little bit easier to review.

What changes are included in this PR?

Are there any user-facing changes?

Not yet since relations can't be used in query yet until #55 gets merged. As a result, tests remains in #55.

[codecov-commenter]

Codecov Report

Merging #547 (b4949d4) into master (8f4078d) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
- Coverage   76.09%   76.08%   -0.01%     
==========================================
  Files         156      156              
  Lines       27048    27051       +3     
==========================================
+ Hits        20581    20583       +2     
- Misses       6467     6468       +1     

Impacted Files	Coverage Δ
datafusion/src/sql/planner.rs	84.75% <75.00%> (-0.11%)	⬇️
datafusion/src/physical_plan/hash_join.rs	84.89% <0.00%> (-0.63%)	⬇️
datafusion/src/physical_plan/expressions/case.rs	75.00% <0.00%> (-0.57%)	⬇️
datafusion/src/physical_plan/planner.rs	77.53% <0.00%> (-0.24%)	⬇️
datafusion/src/execution/context.rs	92.00% <0.00%> (-0.09%)	⬇️
...lista/rust/core/src/serde/logical_plan/to_proto.rs	61.41% <0.00%> (+0.07%)	⬆️
datafusion/src/physical_optimizer/pruning.rs	92.52% <0.00%> (+0.24%)	⬆️
datafusion/src/physical_plan/mod.rs	79.09% <0.00%> (+0.38%)	⬆️
datafusion/src/physical_plan/repartition.rs	86.17% <0.00%> (+1.68%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8f4078d...b4949d4. Read the comment docs.

[alamb]

Makes sense to me (my suggestion is not related to the changes in this PR so please feel free to ignore it)

[Dandandan]

Thanks @houqp looks great