feat: to_pretty_string api

[akoshchiy]

Added to_pretty_string, which is necessary for databendlabs/databend#11270.

[akoshchiy]

@sundy-li @b41sh PTAL.

[b41sh]

LGTM, Thank you for the contribution @akoshchiy

[b41sh]

error: this expression creates a reference which is immediately dereferenced by the compiler
    --> src/functions.rs:1031:17
     |
1031 |                 &pretty_opts,
     |                 ^^^^^^^^^^^^ help: change this to: `pretty_opts`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
     = note: `-D clippy::needless-borrow` implied by `-D warnings`

error: calling `push_str()` using a single-character string literal
    --> src/functions.rs:1064:17
     |
1064 |                 json.push_str("\n");
     |                 ^^^^^^^^^^^^^^^^^^^ help: consider using `push` with a character literal: `json.push('\n')`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_add_str
     = note: `-D clippy::single-char-add-str` implied by `-D warnings`

error: calling `push_str()` using a single-character string literal
    --> src/functions.rs:1115:17
     |
1115 |                 json.push_str("\n");
     |                 ^^^^^^^^^^^^^^^^^^^ help: consider using `push` with a character literal: `json.push('\n')`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_add_str

error: could not compile `jsonb` due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
error: could not compile `jsonb` due to 3 previous errors

please fix the Clippy error @akoshchiy

[akoshchiy]

@b41sh done :)