Formatter options for fmt_eigen

[jwnimmer-tri]

Is your feature request related to a problem? Please describe.

When outputting an Eigen matrix as part of a log message or error text, we might need to control how it gets formatted (e.g., sig figs, leading zeros, comma vs whitespace, transpose, etc).

Describe the solution you'd like

Enhance drake::fmt_eigen to do our own matrix layouts, respecting the format specification mini-language.

(See also https://fmt.dev/latest/syntax.html#range-format-specifications.)

(See also https://eigen.tuxfamily.org/dox/structEigen_1_1IOFormat.html.)

Describe alternatives you've considered

N/A

Additional context

N/A

[jwnimmer-tri]

Planned approach:

• [common] Fix default precision of fmt_eigen floating point #24351 to stop using Eigen::IO so that we have correct precision.
• [common] Provide scalar format strings for fmt_eigen #24349 to obey Scalar format strings (e.g., to allow the user to change float precision).
• [common] Add more decoration to fmt_eigen default layout #24363 to add options for brackets, commas, and/or newlines and change our default to use them.