Implement ToString for ImageIndex, ImageManifest, and ImageConfiguration

[termoshtt]

This PR adds

• to_string and to_string_pretty for three structs listed on title implementing to_writer
  • We can do same thing like

    let image_manifest = ImageManifest::from_file("manifest.json").unwrap();
    let mut out = String::new();
    image_manifest.to_writer(out.as_bytes_mut()).unwrap();

    but to_string is rather simple like to_file is.
• Implement ToString for them
  • Although implementing Display is preferred generally, we cannot avoid twice allocating memory when auto-implementing ToString through Display.

[termoshtt]

Force pushed to fix DCO check.

[codecov-commenter]

Codecov Report

Merging #109 (9885d3d) into main (a4e74fa) will increase coverage by 0.58%.
The diff coverage is 34.00%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   23.56%   24.15%   +0.58%     
==========================================
  Files          20       20              
  Lines        1515     1565      +50     
  Branches      741      768      +27     
==========================================
+ Hits          357      378      +21     
- Misses        514      523       +9     
- Partials      644      664      +20     

[saschagrunert]

LGTM, @utam0k PTAL

[utam0k]

@termoshtt 💯 Thanks!