feat(bsr): Define container and the required fs interfaces

Co-authored-by: Irena Rindos <irenarindos@users.noreply.github.com>

internal/bsr/testdata/TestBsr/session_multiplexed/opened/.journal

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+CREATING SHA256SUM.sig
+CREATED SHA256SUM.sig
+CREATING SHA256SUM
+CREATED SHA256SUM
+CREATING session.meta
+CREATED session.meta
+CREATING session.summary
+CREATED session.summary

internal/bsr/testdata/TestBsr/session_multiplexed/opened/conn_1/.journal

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+CREATING SHA256SUM.sig
+CREATED SHA256SUM.sig
+CREATING SHA256SUM
+CREATED SHA256SUM
+CREATING connection.meta
+CREATED connection.meta
+CREATING connection.summary
+CREATED connection.summary

internal/bsr/testdata/TestBsr/session_multiplexed/opened/conn_1/chan_1/.journal

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+CREATING SHA256SUM.sig
+CREATED SHA256SUM.sig
+CREATING SHA256SUM
+CREATED SHA256SUM
+CREATING channel.meta
+CREATED channel.meta
+CREATING channel.summary
+CREATED channel.summary

internal/bsr/testdata/TestBsr/session_multiplexed/opened/conn_1/chan_1/channel.meta

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+id: chan_1

internal/bsr/testdata/TestBsr/session_multiplexed/opened/conn_1/connection.meta

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+id: conn_1

internal/bsr/testdata/TestBsr/session_multiplexed/opened/session.meta

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+id: session_123456789