Add a ValueOr method to Optional

[bzbarsky-apple]

This allows easier use of the "the value in the Optional or this
default value I have" pattern.

Problem

The pattern for "the value from the Optional or this default I have` is pretty ugly right now.

Summary of Changes

Add a ValueOr method that encapsulates the pattern.

Fixes #3766

[github-actions]

Size increase report for "esp32-example-build" from eda6980

File	Section	File	VM
chip-all-clusters-app.elf	.flash.text	-12	-12

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_info,0,624
.debug_str,0,192
.xt.prop._ZN4chip8BufBound3PutEPKvj,0,40
.xt.prop._ZN4chip8Encoding12LittleEndian7Write32ERPhj,0,-1
.debug_aranges,0,-8
.debug_loc,0,-8
.debug_ranges,0,-8
.debug_abbrev,0,-12
.flash.text,-12,-12
.debug_frame,0,-24
.xt.lit._ZNK4chip8OptionalIyE5ValueEv,0,-40
.debug_line,0,-67

[andy31415]

Code is soo much more readable now.

And somehow 12 bytes less flash. Cool!

[bzbarsky-apple]

@saurabhst @BroderickCarlin @jelderton please take a look?