feat(core): Flexible connection resolution strategy when using read-replicas

[asiFarran]

Allows developers working with read-replicas the flexibility to:

• Set the default resolution strategy for read operations (resolve to a read-replica or write connection)
• Be explicit on a per-query basis by extending FindOptions and CountOptions with a connectionType property

[B4nan]

please remove all unrelated code style changes, and the yarn log file

[B4nan]

Note that we need to keep supporting the string constants (otherwise this would be a breaking change on type level), so remove all of those changes too.

[asiFarran]

Note that we need to keep supporting the string constants (otherwise this would be a breaking change on type level), so remove all of those changes too.

I restored the string constants but did wrap them to a union-type so as to make signatures shorter & 'safer'

[B4nan]

looking good, just two nits

[codecov-commenter]

Codecov Report

Merging #2896 (601d9b8) into master (a4bbebc) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2896   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          194       194           
  Lines        11738     11756   +18     
  Branches      2701      2703    +2     
=========================================
+ Hits         11738     11756   +18     

Impacted Files	Coverage Δ
packages/core/src/drivers/IDatabaseDriver.ts	100.00% <ø> (ø)
packages/core/src/typings.ts	100.00% <ø> (ø)
packages/core/src/utils/Configuration.ts	100.00% <ø> (ø)
packages/core/src/EntityManager.ts	100.00% <100.00%> (ø)
packages/core/src/connections/Connection.ts	100.00% <100.00%> (ø)
packages/core/src/drivers/DatabaseDriver.ts	100.00% <100.00%> (ø)
packages/knex/src/AbstractSqlDriver.ts	100.00% <100.00%> (ø)
packages/knex/src/SqlEntityManager.ts	100.00% <100.00%> (ø)
packages/knex/src/SqlEntityRepository.ts	100.00% <100.00%> (ø)
packages/knex/src/query/QueryBuilder.ts	100.00% <100.00%> (ø)
... and 4 more

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