Add len() to InMemoryWriteableCursor

[mosyp]

Which issue does this PR close?

Resolves the delta-io/kafka-delta-ingest#38

Rationale for this change

There's no efficient way to get buffer length unless copying the data via .data() func. These changes provide easy and safe way top achieve that.

What changes are included in this PR?

Basically the same impl as in .data(), but once we've got the ref then the len() on the underlying is called instead of copying the bytes.

Are there any user-facing changes?

new pub api (?).

[alamb]

Thanks @mosyp

[alamb]

@mosyp it appears clippy has a problem !

https://github.com/apache/arrow-rs/pull/564/checks?check_run_id=3107500562

Can you please fix the error?

warning: build failed, waiting for other jobs to finish...
error: struct `InMemoryWriteableCursor` has a public `len` method, but no `is_empty` method
   --> parquet/src/util/cursor.rs:159:5
    |
159 |     pub fn len(&self) -> usize {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::len-without-is-empty` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

error: aborting due to previous error

[codecov-commenter]

Codecov Report

Merging #564 (593fc7a) into master (f873d77) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #564      +/-   ##
==========================================
- Coverage   82.47%   82.44%   -0.04%     
==========================================
  Files         167      167              
  Lines       46144    46165      +21     
==========================================
  Hits        38059    38059              
- Misses       8085     8106      +21     

Impacted Files	Coverage Δ
parquet/src/util/cursor.rs	78.99% <0.00%> (-4.20%)	⬇️
parquet/src/record/triplet.rs	92.37% <0.00%> (-0.42%)	⬇️
arrow/src/array/array_struct.rs	89.11% <0.00%> (ø)
arrow-flight/src/arrow.flight.protocol.rs	0.00% <0.00%> (ø)

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[mosyp]

Thanks @alamb. I've added the is_empty() to make clippy happy

[alamb]

Thanks @mosyp !