Introduce a Handler hierarchy and DictHandler

[rofinn]

As mentioned in #49

• Create an AbstractHandler type
• Deprecate Handler to JLSOHandler
• Introduce DictHandler which just stores objects in a shared Dict{String, Dict{Symbol, Any}} (vs individual JLSOFiles on disk).
• Loosen existing stage vs commit methods to support both handler options
• Include tests for each of the call-site examples in testpkg.jl

Out-of-scope:

• Generalising the Handler to a FileHandler (the JLSO API is too ingrained)
• Simplifying the stage vs commit behaviour (would require a more significant change to multiple places in the codebase)
• Including any kind of serialization logic to the DictHandler... it's just a field you access
• Support for writing to multiple handlers with the same checkpoint

[codecov]

Codecov Report

Merging #50 (27e02eb) into main (ee797f5) will decrease coverage by 0.12%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
- Coverage   96.42%   96.31%   -0.12%     
==========================================
  Files           5        5              
  Lines         168      190      +22     
==========================================
+ Hits          162      183      +21     
- Misses          6        7       +1     

Impacted Files	Coverage Δ
src/deprecated.jl	100.00% <ø> (ø)
src/Checkpoints.jl	94.33% <80.00%> (ø)
src/handler.jl	95.83% <94.11%> (-1.14%)	⬇️
src/session.jl	100.00% <100.00%> (ø)

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[morris25]

LGTM!