[breaking] Make moi_backend a concrete type and remove MANUAL mode

[odow]

An alternative to #2610

There's a few things going on in this PR because I wanted to see them all together. If we want to do this, we can either merge as-is or break it down into separate PRs.

• Make moi_backend a concrete type
• Rename bridge_constraints to add_bridges
• Encourage add_bridges = false for latency improvements
• Remove MANUAL mode

Closes #2610
Closes #2520
Closes #1627

[codecov]

Codecov Report

Merging #2756 (c393126) into master (6b48409) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2756      +/-   ##
==========================================
+ Coverage   94.18%   94.20%   +0.02%     
==========================================
  Files          43       43              
  Lines        5520     5525       +5     
==========================================
+ Hits         5199     5205       +6     
+ Misses        321      320       -1     

Impacted Files	Coverage Δ
src/JuMP.jl	85.49% <100.00%> (+0.50%)	⬆️
src/copy.jl	95.31% <100.00%> (-0.08%)	⬇️
src/optimizer_interface.jl	77.35% <100.00%> (+1.84%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6b48409...c393126. Read the comment docs.