ARROW-11434: [Rust][DataFusion] Rename length kernel to octet_length

[seddonm1]

This PR renames the length kernel to octet_length to clearly indicate what it returns and allows differentiation from character_length. The use of the term octet could be replaced with bytes but was chosen given there is an ANSI SQL function octet_length.

I have created the correct character_length function as part of #9243.

Issue
The rust length kernel currently counts number of bytes/octets which may or may not be the same as the number of characters given that Arrow uses UTF8 encoding. This means that the result of the length kernel on a string like josé will be 5 bytes rather than 4 characters.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-11434

[jorgecarleitao]

I agree that it may be misleading, but from Rust's perspective, it is not "incorrect" to use length to denote the number of bytes of a string: String::len uses the same convention, and you need to use s.chars().count() to call the number of characters.

This also collides with #9353, where length is extended to support ListArray and BinaryArray.

One idea is to keep the name as is on the arrow crate, but name it octet_length on DataFusion's SQL and API (to be consistent with Postgres).

[seddonm1]

No problem. I will close this PR and raise one with the function comments updated to clarify it's intended behavior.