feat: enable SSL connections

[olavloite]

Adds support for SSL connections to PGAdapter. PGAdapter supports the following three SSL modes:

1. Disabled (default): SSL connections are not accepted. Clients that require SSL will fail to connect. (This is equivalent to the current behavior before this change)
2. Enabled: SSL connections are accepted if the client requests one. Non-SSL connections are also accepted.
3. Required: SSL connections are accepted if the client requests one. Non-SSL connections are denied. This only applies to TCP connections. Unix Domain Sockets are still allowed in this mode (unless Unix Domain Sockets have been disabled).

Running PGAdapter with SSL enabled requires that a keystore with a SSL certificate has been configured. This can be done with the javax.net.ssl.keyStore system property (e.g. -Djavax.net.ssl.keyStore=/path/to/my/keystore.pfx) or by adding the certificate to the default keystore.

[codecov]

Codecov Report

Merging #358 (155eeb0) into postgresql-dialect (7e2fc78) will increase coverage by 0.26%.
The diff coverage is 91.47%.

@@                   Coverage Diff                    @@
##             postgresql-dialect     #358      +/-   ##
========================================================
+ Coverage                 85.96%   86.23%   +0.26%     
- Complexity                 1765     1798      +33     
========================================================
  Files                       120      121       +1     
  Lines                      5863     5955      +92     
  Branches                    800      813      +13     
========================================================
+ Hits                       5040     5135      +95     
+ Misses                      595      593       -2     
+ Partials                    228      227       -1     

Flag	Coverage Δ
all_tests	86.23% <91.47%> (+0.26%)	⬆️

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Impacted Files	Coverage Δ
...ava/com/google/cloud/spanner/pgadapter/Server.java	48.38% <0.00%> (-7.17%)	⬇️
...om/google/cloud/spanner/pgadapter/ProxyServer.java	80.92% <80.00%> (-0.17%)	⬇️
...gle/cloud/spanner/pgadapter/ConnectionHandler.java	84.22% <92.40%> (+5.14%)	⬆️
...gle/cloud/spanner/pgadapter/error/PGException.java	100.00% <100.00%> (ø)
...spanner/pgadapter/metadata/ConnectionMetadata.java	100.00% <100.00%> (ø)
...ud/spanner/pgadapter/metadata/OptionsMetadata.java	81.25% <100.00%> (+1.25%)	⬆️
...panner/pgadapter/wireoutput/AcceptSSLResponse.java	100.00% <100.00%> (ø)
...oud/spanner/pgadapter/wireprotocol/SSLMessage.java	88.88% <100.00%> (+15.55%)	⬆️

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