add epoch field for transactions

[bogdan-rosianu]

Reasoning

• transaction's epoch is exposed by gateway/indexer (even though not all the transactions had this record yet)

Proposed Changes

• extend Transaction structure so the epoch field will exist if fetched from external systems

How to test

[github-actions]

k6 load testing comparison.
Base Commit Hash: 958d9d0
Target Commit Hash: d0dee6f

Metric	Base				Target				Diff
	Avg	Max	90	95	Avg	Max	90	95	Avg	Max	90	95
Nodes	48.07	970.26	50.73	52.53	48.54	355.65	52.75	54.45	+0.99% 🔴	-63.35% ✅	+3.99% 🔴	+3.65% 🔴
Blocks	53.25	1132.00	51.32	53.36	55.42	1444.74	53.22	55.51	+4.08% 🔴	+27.63% 🔴	+3.70% 🔴	+4.02% 🔴
Mex	46.93	147.25	50.90	52.58	48.43	311.19	52.72	54.41	+3.20% 🔴	+111.34% 🔴	+3.58% 🔴	+3.47% 🔴
Transactions	59.40	1376.89	51.46	54.04	62.02	1803.97	53.14	55.30	+4.41% 🔴	+31.02% 🔴	+3.27% 🔴	+2.34% 🔴
Pool	47.58	650.47	50.87	52.68	48.78	568.69	52.75	54.51	+2.52% 🔴	-12.57% ✅	+3.69% 🔴	+3.47% 🔴
Tokens	47.48	502.09	50.94	52.64	48.89	644.46	52.70	54.39	+2.96% 🔴	+28.36% 🔴	+3.45% 🔴	+3.32% 🔴
Accounts	47.78	601.16	50.84	52.32	49.32	888.65	52.55	54.23	+3.24% 🔴	+47.82% 🔴	+3.36% 🔴	+3.66% 🔴
Test Run Duration	60006.56				60002.07

Legend: Avg - Average Response Time, Max - Maximum Response Time, 90 - 90th Percentile, 95 - 95th Percentile
All times are in milliseconds.