loosen type restriction on filename for optsum serialization and rename arg

[palday]

The loosening of the type restriction on the filename-based methods for saveoptsum and restoreoptsum! means that users can pass anytype for which open is defined, e.g. <:AbstractPath.

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away? If so, bump the version:

• I've bumped the version appropriately

[codecov]

Codecov Report

Merging #628 (71593e6) into main (3c192f5) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 71593e6 differs from pull request most recent head 770c3d0. Consider uploading reports for the commit 770c3d0 to get more accurate results

@@           Coverage Diff           @@
##             main     #628   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files          29       29           
  Lines        2765     2765           
=======================================
  Hits         2605     2605           
  Misses        160      160           

Impacted Files	Coverage Δ
src/serialization.jl	93.75% <100.00%> (ø)

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